Method for boride coating steel electrolytically

ABSTRACT

A METHOD FOR IMPROVING THE FORMATION OF A BORIDE COATING ON STEEL BY UTILIZING A HIGHER TEMPERATURE AND A HIGHER CURRENT DENSITY THAN HERETOFORE WHEREBY A FASTER DIFFUSION RATE IS OBTAINED AND POROSITY ELIMINATED.

United States Patent Oflice 3,746,628 Patented July 17, 1973 Int. Cl.C23b 5/00 v US. Cl. 204-39 6 Claims ABSTRACT OF THE DISCLOSURE A methodfor improving the formation of a boride coating on steel by utilizing ahigher temperature and a higher current density than heretofore wherebya faster diffusion rate is obtained and porosity eliminated.

BACKGROUND OF THE INVENTION The formation of a wear resistant coating ona metal composition and in particular a boride coating on steel is knownin the prior art and is described in detail in US. Pat. 3,024,176. Inthe process described in this patent, boride coatings are formed onspecific metals by immersing the selected metal and boron in a fusedbath composed essentially of at least one alkali metal fluoride and atleast one alkali metal fluoroborate so that at least a portion of thebath isolates the metal from the boron. The combination forms anelectric cell in which an electric current is generated when anelectrical connection, which is external to the fused bath, is madebetween the metal and the boron. Under such conditions, the borondissolves in the fused bath and boron ions are discharged at the surfaceof the metal where they form a deposit of boron which immediatelydiffuses into and reacts with the metal to form a boride coating. Therate of dissolution and deposition of the boron is self-regulating sothat the boron is never deposited at a rate faster than it diffuses andalloys with the metal. If a slower rate is desired, it can be easilycontrolled by means well known in the art, such as by the amount ofresistance in the circuit, surface area exposed to the bath, etc. Alimited amount of voltage may be impressed upon the electrical circuitto supply additional direct current if a faster rate is desired.

In the aforementioned patent, it is recommended that in order to producea reasonably fast plating rate and to insure the fusion of the boroninto the metal, the process should be operated in the range of 600 C. to800 C. It is further recommended that the total current density shouldnot exceed 3 amperes per square decimeter and preferably should notexceed 1 ampere per square decimeter after the boride layer is 1 milthick. The reason given is that current densities in excess of theseranges lead to some formation of elemental boron in either the form ofnon-adherent deposits or as granular or large crystalline deposits whichgive a rough undesirable coating.

It is known that porosity in borided coatings presented a technicalproblem. Heretofore, porosity has been controlled by keeping theboriding temperature down to 800 C. When boriding was carried out at 900C., gross porosity developed in the outer portion of the boridedcoating. Since it would be desirable to operate at 900 C. and above totake advantage of the faster diffusion rates at these temperatures, theproblem at hand was to eliminate porosity at these higher temperatures.

SUMMARY OF THE INVENTION In experimenting with the problem of porosity,it was found that porosity occurred at 900 C. because boron diffusedinward by a vacancy mechanism leaving a net flux of vacancies movingtoward the surface. Coalescence of vacancies formed pores at 15 to 25microns from the surface. It was further found that by using a currentdensity of 8 amperes per square decimeter, much higher than hasheretofore been used, a high concentration of boron was maintained atthe surface. The high concentration of boron at the surface created amore rapid diffusion of boron that resulted in vacancies being cancelledbefore they coalesced to form voids.

The ability to produce pore free coatings at temperatures of 900 C. andabove permits one to take advantage of the faster diffusion rates atthese temperatures. Faster diffusion rates allow deeper coatings to beapplied in shorter periods of time. In addition, pore free coatings havea longer wear life that results in savings over coatings containingporosity.

It is, then, a primary object of the present invention to provide anovel and improved method for producing pore free boride coatings.

A further object of the present invention is to provide a novel andimproved method for producing a pore free diffusion coating of boron ona metallic substrate at a faster diffusion rate.

A still further object of the present invention is to provide a noveland improved method for producing a pore free diffusion coating of boronon a metallic substrate which utilizes a temperature of at least 900 C.and a current density of at least 8 amperes per square decimeter wherebya faster diffusion rate is obtained.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Boriding is accomplished byelectrolytic deposition of boron onto the workpiece in a molten fluoridesalt bath. In the electrolytic cell, boron chunks in a copper meshbasket act as the anode and the workpiece is the cathode. When anelectrical potential is applied to the cell, boron plates out on thecathode. The molten fluoride electrolyte operates between 800 C. and1100 C. so that the plated boron diffuses into the workpiece. If theworkpiece is a low carbon steel, two borides of iron form as FeB on thesurface and Fe B beneath it. The hardness of the borides is 1800 Knoopand higher making them suitable as wear surfaces in sliding friction.

The formation of a borided coating takes place in two steps: (1) boronis plated out on the workpiece and (2) boron diffuses into the workpieceto form the intermetallic compounds FeB and Fe B. In order to determinethe mechanism of pore formation, the first fundamental questron to beanswered was whether porosity occurred as a result of the platingprocess or whether it was a diffusion phenomenon.

The approach taken was to see if porosity could be developed independentof the plating process. If this could be done, porosity wouldunquestionably be a diffusion related phenomenon. The experimentationwas done with a piece of AISI 1020 steel borided at 800 C. with a porefree coating. The sample was copper plated .OO2"-.003 and given adiffusion anneal at 950 C. for four hours in a nitroneal atmosphere. TheFeB layer was eliminated by diffusion of boron into the Fe B layer andthe Fe B layer increased in depth. The most important observation,however, was that porosity developed when diffusion took place at 950 C.and it was not present when the sample was originally borided at 800 C.Thus, porosity was developed in a previously pore-free boride coating bya1- lowing diffusion to take place above 900 C. Since this porositydeveloped in a diffusion anneal, independent of the plating process,porosity was clearly identified as a diffusion phenomenon.

Having established that porosity was related to a diffusion phenomenon,two mechanisms of pore formation were hypothesized. First, a defectstructure developed as a means of accommodating a boron concentrationgradient in the stoichiometric compounds FeB and Fe B which have nohomogeneity range. The second hypothesis was that boron diffused on theboron sub-lattice by a vacancy mechanism in both the FeB and Fe B phaseswith no diffusion of iron taking place. This created a flux of vacanciesmoving toward the surface which coalesced to form porosity.

Taking the first hypothesis, it was noted that the phase diagram for theiron-boron system showed that both FeB and Fe B have no homogeneityrange. They were depicted by a single vertical line at 33 atomic percentboron and at 50 atomic percent boron. If diflusion was driven by a boronconcentration gradient, this gradient may have been accommodated by adeftct structure consisting of vacancies, in excess of the equilibriumconcentration of vacancies, at the low boron side of the phase boundarybetween FeB and Fe B. This situation would be analogous to the formationof vacancies on the aluminum rich side of the stoichiometric compoundNiAl.

To determine if a boron concentration gradient existed in thestoichiometric compounds FeB and Fe B, electron microprobe traces wererun on borided AISI 1020 steel. Traces were obtained by driving thesample across a stationary beam at 4 microns per minute. A chart was runat 0.5 inch per minute so that 10 small divisions on the chartrepresented 8 microns. The traces showed two plateaus of boron, one inthe FeB region and another in the Fe B region. The boron concentrationin both the FeB and Fe B phases appeared to be level with noconcentration gradient present. In the absence of a boron concentrationgradient, abrupt changes in boron concentration occurred at the phaseboundaries. There being no boron cncentration gradient in either the FeBor Fe B phases, the hypothesis that defect structures occurred byaccommodation of a concentration gradient in a stoichiometric compoundwas rejected.

The second hypothesis stated that boron diffused on the boronsub-lattice by means of a vacancy mechanism in both the FeB and Fe Bphases with no diflusion of iron taking place. Iron simply expanded fromits bcc or foe lattice, depending on boriding temperature and carboncontent, to the body centered tetragonal lattice of Fe B when the boronconcentration reached 33 atomic percent and to the orthorhombic latticeof FeB when the boron concentration reached 50 atomic percent. Whenboron diffused inward by a vacancy mechanism and no diffusion of irontook place, it created a flux of vacancies moving toward the surface.Vacancies coalesced to form voids which showed up as gross porosityunder metallographic examination. Implicit in this hypothesis was theproposition that porosity could be eliminated by increasing thediffusion rate of boron near the surface so that the vacancies movingtoward the surface would be cancelled before they coalesced to formvoids and porosity. Increasing the diffusion rate of boron near thesurface could be accomplished by increasing the concentration of boronat the surface by means of a higher current density.

In carrying out the boriding process with a higher current density,General Electric metalliding equipment was used which comprisedessentially an Inconel vessel fitted with a Monel liner. Into the vesselwas placed a molten fluoride electrolyte having a composition of 39.3Weight percent LiF, 28,8 weight percent NaF and 31.9 weight percent KF.The vessel was covered with a cover plate which contained two ports forelectrodes and one port for a thermocouple well and one port forconnection to a bubbler tube. Two glass towers are fitted over theelectrode ports with connections to a vacuum source and/ or a protectivegas atmosphere. A copper mesh basket containing chunks of boron wasanode bound by copper wire to a tungsten rod sealed at the top of oneelectrode tower with rubber tubing. A similar arrangement was used for al" x 1" x .040" strip of AISI 1010 low carbon steel cathode in the otherelectrode tower.

The vessel was heated by electrical resistance and brought to atemperaturue of 900 C. under a forming gas nitrogen, 10% hyrogen)atmosphere. A current from a constant current power supply was impressedon the electrolytic circuit by way of an external electrical circuitconnecting the anode and the cathode to maintain a current density of 8amperes per square decimeter on the cathode. The steel strip wasimmersed half way into the bath and was borided for 4 hours at theconstant current density of 8 amperes per square decimeter. An ammeterand voltmeter were connected in the normal way in the external circuit.The pertinent data was:

Current Temp Density, Voltage,

Time, min amps/dm 1 Amps volts After the desired time duration of therun is completed, the circuit is turned off, the workpiece is lifted outof the molten salt'bath and cooled under forming gas atmosphere in theelectrode tower.

A photomicrograph for the above run showed a deep, dense, pore-freecoating having a thickness of 5 mils. Similar runs were made at 900 C.using current densities of 2.5 and 4.0 amperes per square decimeter andin both cases the resulting coating was shallow and porous. The seriesof samples showed that as current density increases the depth of coatingalso increases when the time and temperature are held constant. Theability to produce deeper coatings is an additional benefit that occurswhen the current density is raised to 8 amperes p r square decimeter toeliminate porosity.

The high current density produced a lot of fine particles of elementalboron on the surface of the workpiece as expected. The boron particleswere interspersed with the fluoride salts and were easily removed with atreatment in boiling water after removal of the workpiece from theelectrolytic equipment. The resulting surface was smooth and the boridecoating was adherent and pore free.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is: 1. A method of forming a boride coating on a lowcarbon steel workpiece which comprises;

forming an electric cell containing a molten fluoride electrolytecomposed essentially of lithium fluoride, sodium fluoride and potassiumfluoride and having said steel workpiece as the cathode joined throughan external electrical circuit to a boron anode;

maintaining said electrolyte at a temperature of at least 900 C. in theabsence of oxygen;

maintaining a current flowing in said electric cell so that the currentdensity at the cathode will not be less than 8 amperes per squaredecimeter during the formation of the boride coating; and

stopping said current flow after a desired time duration of boriding iscompleted whereby a pore free boride coating of a desired depth isobtained in a shorter time by using a high current density.

2. A method of forming a boride coating on a low carbon steel workpiecewhich comprises:

forming an electric cell containing a molten fluoride electrolytecomposed essentially of 39.3 weight percent lithium fluoride, 28.8weight percent sodium fluoride and 31.9 weight percent potassiumfluoride and having said steel workpiece as the cathode joined throughan external electrical circuit to a boron anode;

maintaining said electrolyte at a temperature of at least 900 C. in theabsence of oxygen;

maintaining a current flowing in said electric cell so that the currentdensity at the cathode will not be less than 8 amperes per squaredecimeter during the formation of the boride coating; and

stopping said current flow after a desired time duration of boriding iscompleted whereby a pore free boride coating of a desired depth isobtained in a shorter time by using a high current density. 3. A methodof forming a boride coating on a low carbon steel workpiece whichcomprises:

forming an electric cell containing a molten fluoride electrolytecomposed essentially of 39.3 weight percent lithium fluoride, 28.8weight percent sodium fluoride and 31.9 weight percent potassiumfluoride and having said steel workpiece as the cathode joined throughan external electrical circuit to a boron anode; maintaining saidelectrolyte at a temperature of 900 C. in the absence of oxygen;

maintaining a constant current flowing in said electric cell so that thecurrent density at the cathode will be 8 amperes per square decimeterduring the formation of the boride coating; and

stopping said current flow after a desired time duration of boriding iscompleted.

4. The method of claim 3 wherein said current flow is stopped at thecompletion of 4 hours of boriding whereby a pore free boride coating ofa desired depth is obtained in a shorter time by using a high currentdensity.

5. A method of forming a boride coating on a low carbon steel workpiecewhich comprises:

forming an electric cell containing a molten fluoride electrolytecomposed essentially of 39.3 weight percent lithium fluoride, 28.8weight percent sodium fluoride and 31.9 weight percent potassiumfluoride and having said steel workpiece as the cathode joined throughan external electrical circuit to a boron anode;

maintaining said electrolyte at a temperature of 900 C. in the absenceof oxygen;

maintaining a constant current flowing in said electric cell so that thecurrent density at the cathode will be 8 amperes per square decimeterduring the formation of the boride coating;

stopping said current flow after 4 hours of boriding is completed; and

removing the workpiece from the cell and treating it in boiling water toremove boron particles and fluoride salts from the surface whereby apore free boride coating of a desired depth is obtained in a shortertime by using a high current density.

6. A method of forming a boride coatingon a low carbon steel workpiecewhich comprises:

forming an electric cell containing a molten fluoride electrolytecomposed essentially of lithium fluoride, sodium fluoride and potassiumfluoride and having said steel workpiece as the cathode joined throughan external electrical circuit to a boron anode;

maintaining said electrolyte at a temperature of at least 900 C. in theabsence of oxygen;

maintaining a current flowing in said electric cell so that the currentdensity at the cathode will be greater than 4 amperes per squaredecimeter during the formation of the boride coating; and

stopping said current flow after a desired time duration of boriding iscompleted whereby a pore free boride coating of a desired depth isobtained in a shorter time by using a high current density.

References Cited UNITED STATES PATENTS 3,024,176 3/1962 Cook 204-39TA-HSUNG TUNG, Primary Examiner R. L. ANDREWS, Assistant Examiner

